1. Field of the Invention
The present invention relates to a noise suppression apparatus for an FM radio receiver in which incoming noise, particularly pulse noise, is removed.
2. Prior Art
FIG. 5 shows a prior art pulse noise removing apparatus for the FM radio receiver. An FM-detected signal which is outputted from an FM detection circuit 1 of the receiver is supplied to a delay circuit 2 in the form of an LPF (low-pass filter) where the signal is delayed by a predetermined length of time. The output of the circuit 2 is supplied to an MPX circuit (multiplex stereo demodulation circuit) 5 through a gate circuit 3 and then a level-hold circuit 4. The delay circuit 2 is for providing the signal with a delay time equal to the time required for the signal supplied to an HPF 6 to exit a shaper 9. The FM-detected signal is supplied to the HPF 6 (high-pass filter) for noise detection. The noise component through the HPF 6 is amplified by a noise amplifier 7 and is then supplied to a noise detection circuit 8.
The noise detection circuit 8 includes a rectifying circuit for rectifying the output of the noise amplifier 7, and a level comparator circuit for comparing the output level of the rectifier with a predetermined reference level. The output of the noise detection circuit 8 is supplied to the shaper 9. The shaper 9 is, for example, in the form of one shot multivibrator which converts the noise detection output into a pulse having a predetermined height and a predetermined width and sends the pulse to the gate circuit 3. The gate circuit 3 is driven by the pulse from the shaper 9 into a closed condition where the delayed output level just before the gate circuit 3 is closed is held by the level-hold circuit 4 and is sent to the MPX circuit 5. By this arrangement, the pulse noise is removed from the signal before it is supplied to the MPX circuit 5. The output signal of the noise amplifier 7 is rectified by a rectifier 10 the output of which is then supplied to an LPF (low-pass filter) 11. The rectified output is smoothed out by the LPF 11 to provide a d-c voltage in accordance with the noise level and feeds back to the noise amplifier 7. The thus described structural elements 7, 8, and 11 form an AGC loop for the noise.
With the FM receiver, white noise increases as the incoming r-f signal level becomes weaker, in which case the noise level in the output signal of the HPF 6 increases. Therefore higher the noise level is, the higher is the d-c signal level as an AGC signal outputted from the LPF 11. The higher d-c level causes the gain of the noise amplifier 7 to go down. Thus, the noise level supplied to the noise detection circuit 8 from the noise amplifier 7 is maintained at a substantially constant level, thereby preventing the saturation of the noise amplifier 7. In this type of pulse noise removing apparatus, the high frequency component in the FM-detected output is rectified and then smoothed out to produce the d-c signal which indicates the average level of the high frequency component of the FM detected output. Any amplitudes greater than that average level are detected as pulse noise to control the gate circuit to close for noise removal.
Removing the pulse noise alone is not sufficient to suppress the noise to an acceptable level. Various characteristics of the FM broadcast wave are susceptible to variations in terrain. Particularly, on-vehicle receivers experience this kinds of changes in characteristics of the FM broadcast wave. Thus, skip noise or intermittent noise and multipath noise affect the signal to result in a significant deterioration in signal-to-noise ratio particularly when the signal strength is weak.
For example, Japanese Utility Model Publication No. 5931077 shown in FIG. 6 discloses an FM stereo receiver capable of overcoming the above-described drawbacks. Elements similar to those in FIG. 5 have been given the same reference numerals.
In FIG. 6, an r-f signal through antenna 12 is fed to a front end 13 where the r-f signal is mixed with the local frequency from a local oscillator 14 to produce an intermediate frequency. The intermediate frequency is amplified by an i-f amplifier 15 and is then frequency detected by an FM detection circuit 1. The detected output passes through a variable attenuation circuit 16 in the form of a variable attenuator circuit to an MPX (stereo demodulation) circuit 5 where the signal is separated into the left and the right signals. The left and right signals are then controlled their separation by a separation control circuit 17.
The left and right signals are then supplied to a high frequency characteristic control circuit 18 to be cut their high frequency component before they are outputted to the output terminals. The i-f amplifier 15 is followed by an amplifier 19 for amplifying the intermediate frequency signal, a detector 20 for envelope-detecting the output of the amplifier 19, and an LPF (low-pass filter) 21 in series. The d-c signal provided by the LPF 21 is applied to a first, second, and third level setting circuits 22, 23, and 24 each of which outputs a control signal. The control signal from control signal from the first level setting circuit 22 is supplied to a separation control circuit 17 which mixes the left signal and the right signal by gradually increasing the level of one of the signals relative to the other signal. Since the noise component in the left and right channels are opposite in polarity, superimposing the two signals together causes the noise component in the two channels to cancel out each other while simply superimposing the audio signals. As the signal level that is added to the other signal is increased, the stereo signal outputted from the output terminals OUT will be increasingly poor in stereo separation, becoming more like a monaural signal. The control signal from the second level setting circuit 23 is supplied to a high frequency characteristic control circuit 18 to gradually attenuate the high frequency component in the left and right signals as the r-f signal level decreases. The control signal from the third level setting circuit 24 is supplied to the variable attenuation circuit 16 to progressively decrease the input level to the MPX circuit 5 in accordance with the level of the r-f signal level decrease.
Thus, as the r-f signal strength decreases, the separation control circuit 17 first operates to switch its operating mode from the stereo mode to the monaural mode to prevent the increase in noise. Then, the frequency characteristic control circuit 18 goes into operation to prevent a further increase in noise as the r-f signal strength further decreases. When the noise level tends to increase even further, the variable attenuation circuit 16 operates to smoothly decrease the signal level supplied to the MPX circuit 5. The sequential operation of the respective control circuits 22, 23, and 24 described above reduces the virtual noise to a reasonably low level.
The aforementioned pulse noise suppressing means and the noise suppressing means which includes the separation control, exist separately, and the d-c control signal for controlling the operations of these means are provided by separate circuits. This leads to the problem of a greater complexity of circuit configuration.